A large variety of “thin films” are used in the fabrication of microelectronic devices. For example, these films may be thermally grown or deposited from a vapor phase and may include metals, semiconductors or insulators. Due to the extremely fine patterns and dimensions of features that are formed in a microelectronic device, the thickness of the films and the depth of the patterns etched therein are significant factors in achieving acceptable product yield.
Uniformity of thin film thickness and precise etching depth control are particularly critical in etching processes. Previously, film thickness and feature depth were determined ex-situ after the etching processes by optical measurement before and after plasma processing of known duration. This process was not suitable as a diagnostic tool for real-time process control, i.e., in-situ monitoring and control of the etching process.
Accordingly, a number of in-situ techniques now exist to measure etching depth and/or film thickness. One such technique, optical emission interferometry, analyzes the interference of light reflected from a thin film being etched or deposited. Generally, monitoring film thickness or determining an etch depth progression by interferometry involves selecting a wavelength λ for irradiation incident on a layer to be etched, measuring an index of refraction n of the layer, and collecting optical signals reflected from irradiation incident on the layer. The optical signals are then analyzed to separate the frequency f of the layer being etched. A wave number N may then be determined by the equation:N=f*Δt;  (1)where Δt is a predetermined time interval, such as a simulated etching processing time interval. Assuming the incidence of the irradiation is substantially normal to the surface of the layer being etched, the etch depth D may be subsequently determined by the equation:D=N*λ/(2*n).  (2)
Those skilled in the art will recognize that film deposition thickness may be determined in a similar manner.
While this conventional method provides satisfactory prediction of etch depth and deposition thickness, it relies on the assumption that the layer being processed has substantially uniform characteristics, such as a uniform index of refraction (n). However, many layers employed in existing microelectronic devices, such as extremely low-k (ELK) dielectric layers, comprise multiple layers of different materials having different refractive indices and other characteristics. For example, process requirements for managing stress values related to ELK layers while maintaining desired dielectric values may mandate a multi-layer film in which the refractive indices of the individual layers may vary by 25% or more. Thus, it follows from equation (2) above that any inaccuracy of the index of refraction employed to monitor the etch progression can result in a corresponding inaccuracy in the resulting layer thickness or etch depth.
Accordingly, what is needed in the art is a method of manufacturing microelectronic devices having multi-layer films of varying refractive indices or other varying characteristics that addresses the problems discussed above.